Thermodynamics of SiO2–H2O fluid near the upper critical end point from quartz solubility measurements at 10 kbar

Quartz solubility in SiO2–H2O fluid was measured at 10 kbar and 750–1130 °C in a piston-cylinder apparatus. At ≤ 1035 °C, solubility was determined by weight loss of single crystals equilibrated with H2O-rich fluid; at higher temperature (T), the fluid phase coexisting with quartz quenched to a glass, and solubility was determined by phase-assemblage bracketing (quartz present or absent with glass in quenched charges). At 700 to 1000 °C, quartz solubility increases at an accelerating rate from 1.2 mol% to 8.9 mol% SiO2. Above 1000 °C, there is a sharp increase to nearly equimolar fluids in equilibrium with quartz at 1080 °C. At T > 1080 °C, SiO2 concentration increases less strongly. The data confirm the existence of a critical end point on the hydrous melting curve of quartz, and imply it lies at 1080 °C and 9.5–10 kbar. Two independent approaches to calculating SiO2 activity at 1080 °C — from a mixing model for aqueous SiO2 extrapolated from lower temperatures, and from depression of the melting temperature of quartz by H2O — indicate nearly constant values over the wide compositional range of 20–60 mol% SiO2. The activity of H2O (ah) at the same conditions, calculated by integration of the Gibbs–Duhem relation, shows a plateau of very high ah (∼ 0.93) in the same broad composition range. At ≥ 60 mol%, there is an abrupt change in H2O dissolution mechanism, as SiO2 activity becomes proportional to its mole fraction at high silica concentration. The activity-concentration relations at 1080 °C and 10 kbar were fitted to a subregular solution model, giving interchange energies for SiO2 and H2O of respectively 25.3 and 14.3 kJ/mol. Roughly constant SiO2 activity of the near-critical fluid despite a factor of 3 increase in XH2O can be explained by progressively greater amounts of hydrogen bonding of H2O molecules to polymerized silica units, so that the effective concentration of these units remains nearly constant as H2O increases. The sudden onset of critical phenomena in the system SiO2–H2O as temperature and pressure increase over narrow intervals approaching 1080 °C and 10 kbar is thus explained as mainly a convergent or cooperative compositional effect, in which polymerization and hydrogen bonding both play important roles.